Coal usage has increased in the United States for a variety of reasons, particularly those of an economic nature. The utility industry is burning far more coal today than it did ten years ago. With the increased demand for coal, the use of younger, more volatile coals like subbituminous and lignite has increased. Consequently, the potential for spontaneous combustion causing serious fires and explosions during the handling, grinding and pulverizing steps has increased.
Several methods which have been given considerable attention for detecting impending pulverizing mill fires are based on measuring temperature, gas flow velocity and carbon monoxide. Single and multiple point temperature monitoring techniques have been used for a number of years to warn of an over-temperature condition in the mill. This approach, however, provides information too late to stop a fire from spreading. The gas flow velocity monitor approach has potential, but the relationship between gas flow, temperature and pressure are not sufficiently understood to be effective as a warning system. The increase in the carbon monoxide level in the pulverizing mill has been recently given the most attention in research and practice and is a way of detecting pulverizing mill fires.
A number of commercial devices utilizing infrared absorption techniques are available for monitoring carbon monoxide levels in the pulverizing mill. This method is based upon the principle that when coal starts to oxidize, i.e., the early stages of combustion, carbon monoxide is produced. Being able to detect this carbon monoxide at very low levels, e.g., 25 to 50 ppm, permits the mill operator to take precautionary measures to prevent a major fire or an explosion in the mill.
A small pocket of oxidizing coal can become a major fire through escalation or ignition. If escalation occurs, the oxidation process intensifies as the quantity of coal involved and temperature increase. Larger quantities of carbon monoxide are produced as the process escalates until a runaway condition is reached which results in a fire. This small quantity of oxidizing coal also represents an ignition source which combined with the other elements within the mill can result in a major fire or explosion. In this case, the quantity of carbon monoxide does not need to escalate prior to the fire or explosion since the small pocket of oxidizing coal is only an ignition source. From the foregoing, it is apparent that detection methods based upon carbon monoxide alone are useful only after oxidation has started and do not give the operator a good indication of potentially explosive conditions within the pulverizing mill. Other factors, such as the level of oxygen and combustible gases in the pulverizing mill, must be considered when evaluating the possibility of a fire or an explosion within the pulverizing mill.
Because of the foregoing it has become desirable to develop an improved safety control system for a pulverizing mill wherein a measurement of oxygen and an aggregate measurement of not only carbon monoxide but all combustible gases in the mill are made and utilized for controlling the operation of the mill and warning the operator of a potentially dangerous mill condition.